Method of making metal flakes



Unite States Patent l\ IETHOD OF MAKING METAL FLAKES Walter R. McAdow, Grosse Pointe Farms, Mich, as-

signor, by mesne assignments, to American-Marietta Company, Chicago, Ill., a corporation of Illinois No Drawing. Application April 15, 1955 Serial No. 501,731

13 Claims. c1. 75-5 This invention relates to methods for producing metal mirror-plate or mirror-flake particles of extreme thinness, preferably from about three millionths of an inch to about twenty-five millionths of an inch in thickness, said particles being individually transparent to the naked eye by virtue of their extreme thinness. It also relates to suspensions of said mirror-plate particles in a relatively inert liquid suitable for thinning paint formulations.

The mirror-plate particles made by the methods of the present invention exhibit intra-molecular light transmission by virtue of their transparency in the extreme thinness of the particles, and further exhibit very high light reflectance or surface luster by virtue of the metal material employed and in view of the particular conditions which are observed during the preparation of these particles. Because of this unusual combination of light reflectance and light transmission properties and because of the very large surface area presented in a unit of weight or of volume of such particles, the mirror-plate particles are particularly suitable for the preparation of metallized coating compositions containing a suitable film-forming material for dispersing the particles in the film which is applied to a base.

The coating compositions embodying the use of the mirror-plate particles as produced herein and otherwise, are described in detail in my copending application, Serial No. 494,822, filed March 16, 1955.

In accordance with the invention, mirror-plate particles having a thickness of about 3 millionths of an inch to about millionths of an inch and which are transparent under 'visible light due to their extreme thinness are produced by depositing a high light-reflecting metal in a continuous film of uniform thickness up to about 25 millionths of an inch on to a coated base, the base being uniformly and continuously coated with a smooth, metalreleasing layer, and releasing said metal film from the base by treating the assemblage with a liquid which is relatively inert to the deposited metal film but which softens or dissolves the metal-releasing layer. Thereafter the released metal film is agitated in said inert liquid to break up the continuous metal film into metal particles of a desired particle size.

The production of transparent mirror-plate particles of about 3 to 25 millionths of an inch in thickness may be improved in accordance with the invention by alternately depositing a light-reflecting metal film of the required thickness and a metal-releasing layer onto a base coated with said metal-releasing layer until two, three or even more alternating metal-releasing layers and a like number of metal films are assembled on the base, and thereafter removing the metal films from the base by treating the assemblage with a liquid to soften or dissolve the metal-releasing coatings and release the metal films, and breaking up the released metal film in the liquid to produce metal particles of a desired particle size.

Further in accordance with the invention, the mirrorplate particles of about 3 to 25 millionths of an inch in thickness may be suspended in a relatively inert liquid ice suitable for thinning paint formulations and the liquidparticle combination may then be standardized to provide a constant weight of particles of uniform covering power in the said liquid.

Other and further advantages of the present invention will appear from the more detailed description set forth below, it being understood that such more detailed description is given by way of illustration and explanation only, and not by way of limitation, since various changes may be made by skilled workers in the art without departing from the scope and spirit of the present invention.

Ordinary aluminum flake metal has been in use for some years as a pigment in paints, enamels, lacquers and other coating compositions. The various grades of fineness of aluminum flake for pigment purposes varies from relatively large particle sizes, such as about 60 mesh to very fine particle sizes, less than 325 mesh.

The commercial aluminum flake is usually made by stamping or milling aluminum sheet of appropriate brightness, purity and having suitable mechanical properties which promotes uniformity in the cut product, after which the flakes are graded and collected.

By suitable known treatments, the commercial aluminum flake pigment is made to leaf, and thereby improve the hiding power of the pigment.

It has been thought that a necessary requirement of ideal particle size of the aluminum pigment, either of leafing or nonleafing type, is that the individual particles have no dimension less than .005 millimeter or 0.0002 inch.

For the finest graded pigment finishes, the enamel formulators have been satisfied to approximate the socalled ideal by removing from 60 to of particles smaller than 200 millionths of an inch. The commercial product made by milling the particles to sizes less than 200 millionths of an inch is known to impart a gray, milky appearance and impair brightness to the finished enamel film. These fine particles are removed to prevent the destruction of the clear clean basic color of the enamel which is highly desirable in the finish.

Accordingly, the most important commercial use of aluminum flake, the use in the paint industry, has caused the flake manufacturers to concentratetheir efforts in a particle thickness range of above 200 millionths of an inch. It is surprising to discover, contrary to judgment and technical knowledge in the prior art, that particle sizes of the metal flakes in the range of 3 to 25 millionths of an inch, preferably about 5 millionths of an inch to about 15 millionths of an inch, produce wholly unexpected and beneficial light transmitting and light reflecting effects which produce various combinations of luster, brilliance, color covering power and depth not heretofore observed. The methods of making these particles as disclosed and claimed in the present invention satisfy the need for a practical technical method for making this very desirable product.

Metal-mirror particles obtained by breaking up vapor plated metal deposited in thicknesses of about 3 to 25 millionths of an inch, preferably about 5 to 15 millionths of an inch, on smooth surfaces are particularly adaptable for incorporating into coatings having high reflective characteristics. Those skilled in the art and science of vapor deposition of metals believe that the brightness and brilliance is due to the fact that the vapor deposited metal is so thin that oxidation is very minute and it is believed that the metal surface is protected by an oxidized film which appears to be very minute and quite different in character than the usual oxide film. The oxide film, extremely thin and but a fraction of the deposited metal film, is thought to be a pure, smooth, transparent oxide crystal whereas in regular conventional types of aluminum particles the surfaces are rough.

The conventional metal particles being thicker oxidize more deeply, in strata, and are consequently duller. The conventional fine particle, such as aluminum, is more opaque and does not permit light to be transmitted through the particle. It has been noted in angular observation of colored panels containing these mirror-plate particles prepared by the method of the invention that they show a brilliance in coating compositions much greater than the conventional metal particles.

The fiat surface dimensions of the metal mirror-plate particles vary widely, but in the main they are of a size that can pass readily through a 300 mesh size screen. By continued agitation in xylol or other coating solvent the size of the particles can be made finer than a 350 or a 400 mesh. These very fine particles, especially those finer than a 400 mesh, are preferred for spray applications.

To produce these flat surface dimensions of 300 to 400 mesh inmetal mirror-plate particles from about 3 to 25 millionths of an inch in thickness, it is necessary that the thin metal film which is applied to the base be stripped from the base in a condition such that it can be broken up to the desired size without producing too many fines and that the sized particles still retain the beneficial light transmission and light reflectance properties which are essential characteristics of the product.

Thin metal films suitable for forming mirror-plate particles of the order of about 3 to 25 millionths of an inch in thickness may be made by various methods which include:

(1) Mechanical methods: (a) Rolling or beating, such as with gold foil (2) Chemical electro chemical methods:

(a) Electrodeposition, such as with chromium (b) Reduction, such as with silver mirrors (3) Physical methods: (a) Thermal evaporation such as aluminum, etc.

The mechanical methods of rolling or beating to produce a foil. generally adversely affect the reflectivity so that additional brightening treatment of the foil is required. The mechanically beaten foil is also extremely difficult to prepare in uniform thickness. Further, only relatively few metals such as gold may be mechanically beaten to the film thickness as required in the present invention. As a practical matter, the difficulties involved in economically producing the starting metal such as aluminum are so great, that this method of treatment is not of general suitability in accordance With the invention. In fact the minimum thickness of rolled aluminum is of the order millimeters, which is far greater than that required in the present invention.

As a production method, the thermal evaporation process (vapor deposition) is preferred, since with commercial metals, such as aluminum, it is difficult to beat or roll aluminum to less than .001 millimeter, and except for metals like chromium, electrodeposited films are usually too thick. Chemical reduction methods are more diflicult to control and more expensive.

Generally, methods of thermal evaporation are well known, see Thin Films and Surfaces by Winifred Lewis, Chemical Publishing Company, Inc., 1950, at pages 3746. Substantially every metal may be deposited on a base by these latter methods and of these metals, the metals which have high reflectivity, intra molecular transparency and the necessary properties of inertness in the coating vehicle after being separated from the base and flaked, are preferred. These reflectivity and transparency properties as well as other properties have been described for various wave lengths (see Thin Films and Surfaces).

Ordinary electrodeposited coatings because of their greater thickness cannot be utilized in producing the desired mirror-plate particles having a thickness of 3 to 25 millionths of an incha However, using chromium,

bright electrodeposited films of the order of 15 to 20 millionths of an inch in thickness have been produced by imposing special conditions in accordance with the invention and in which the chromium film is deposited on a smooth organic solvent soluble parting or releasing layer of uniform thickness over the base and thereafter separated and broken up into the desired mirror-plate particle size by treating the base, parting layer and deposited film with a solvent for the parting layer While gently agitating the released film to break it up.

The thin chromium films produced by electrodeposition are somewhat thicker (15 to 20 millionths of an inch) than thin films produced by vapor deposition under vacuum (about 3 to 5 millionths of an inch). It is believed that the greater thickness of the electrodeposited films at least partly explains the lower light reflectance values and the reduced light transmission values of electrodeposited films as compared with vapor deposited films. The reflectivity of electrodeposited chromium in thick films is as high as 62%, but this reflectivity is reduced somewhat in the thinner films. In vapor deposited chromium films of thicknesses of the order of about 5 millionths of an inch, larger reflectivity values are obtained and better light transmission characteristics are observed.

In general, any metal mirror-plate particles of thickness about 3 to 25 millionths of an inch and having a light reflectivity of at least 35% in the visible light range are desirable products to be produced by the methods of the present invention. With such metals as vapor deposited aluminum, light reflectivity of the order to is observed in these thicknesses and a light transmission of the order of /2 to 1 /2 is observed with thinner films. Although there does not appear to be any relationship between reflectance and transmission values, and although each of these change with wave length, it has been found that beneficial light transmitting effects are obtained with the known highly reflective metals, particularly as these are used in metallized paint, enamel, lacquer and other coating compositions. The method giving the best results is the vapor deposition method, but good results providing beneficial coating formulations are obtained also with cathode sputtered films and with electrodeposited chromium films which are treated by the method of the present invention.

Examples of the vapor deposition of aluminum and clectrodeposition of chromium are now described.

(A) Vapor deposition method The basic process applies a thin metal film to a base in a very high vacuum. Only three operations are necessary:

(1) The base is given a strippable organic coating to provide a mirror-smooth surface to carry the thin metal layer;

(2) The metal film is applied in a thickness of about 5 to 25 millionths of an inch in the high vacuum chamber of evaporation of the metal from a heated filament; and

(3) The metal film is stripped from the base and broken up mechanically into particles.

The metal that is to form the coating film is evaporated at high temperature near the center of the vacuum vessel in such a way that the evaporated metal traveling in straight lines strikes the surface to be coated. Although the filament is hot enough to boil aluminum, the Work stays at about the temperature of the room. The extraordinary high vacuum (less than 1 micron of mercury) is needed to allow the metal to evaporate at temperatures that can be attained reasonably with a hot filament and also to permit the infinitesimally small particles of the evaporated metal molecules to travel over the considerable distance necessary to reach the work without in the meantime having been impeded by hitting molecules of the air filling the space.

By a series of high speed vacuum pumps, the density of the air molecules in the space between the evaporating metal and its target is reduced to a millionth or even a ten-millionth of its original value and then a great many metal molecules, freed by evaporation, can hit and stick to the work.

Although decorative electroplates are sometimes only a tenth of a mil thick, films common in vacuum metallizing are far thinner, usually only a few thousands of a mil thick. The preferred metal for vacuum metallizing is aluminum, because it readily vaporizes under practicable conditions and because the film formed from it possesses high reflectance (85% average) in the thin layers employed.

Metallizing the film to the base from which the film is to be stripped is carried out in large cylindrical coating chambers, as much as 5 /2 feet in diameter by four to six feet long. The cylinders are placed horizontally with one end open to allow the base to be put in on fixtures.

Diffusion pumps connected to the chamber may be backed up by mechanical pumps. Comparatively heavy tungsten filaments arranged along the axis of the cylindrical coating chamber are connected to an electric power supply that permits them to be heated to high temperatures in a few seconds.

On each section of the tungsten wire are hung small pieces of the metal to be used for coating, such as aluminum, platinum, silver, copper, gold, tin, palladium, titanium, zinc, antimony, nickel, etc., in such a way that this will be melted and heated to the vaporizing point by the current in the tungsten wires. Any metal may be used capable of being vapor deposited under vacuum in the thickness range of about 3 to 25 millionths of an inch. The vacuum pumps are started.

As soon as the pressure has been brought to the desired low point (less than one micron of mercury absolute pressure), the filaments are heated to evaporate the metal. The jigs containing the base expose the surface of the base to the stream of metal vapor pouring from the hot filaments.

The vacuum in the coating chamber is relieved and the base removed, still on its fixture. The cycle in the metallizing chamber consumes fifteen to thirty minutes,

from loading the work into the chamber until it is removed. The work is kept dry and free from solvents that would evaporate in the vacuum. If the relative humidity of the work room is low so that no moisture is absorbed or condensed on the surface of the work, the time required for reaching the requisite vacuum is materially shorter than when these conditions do not prevail.

The evaporated metal film is not quite as dense as the massive metal and the mirror-plate particles, say those about 5 millionths inch thick which is the preferred particle thickness in accordance with the present invention, can be penetrated by moisture and solvents. For this reason, the mirror-plate particles are handled as a suspension in an inert solvent, compatible with the filmformer when added to an organic film-former.

A typical example of the removal of the vapor platedv metal for coatings is given below.

Aluminum panels 5 inches x 16 inches were coated with a workable'automotive black lacquer on both sides. A second set of ten 5 inches x 16 inches aluminum panels were coated with a release coating in the form of a. maleic anhydride modified ester gum resin solution on both sides and allowed to air dry. These panels were placed in a vacuum plating chamber and were vacuum plated with aluminum. The resin coated panels showed a smoother, brighter plating whereas the panels coated with black lacquer were not as smooth or as bright 'ndicating that some of the components in the black. acquer were removed from the film in the vacuum hamber. The preferred release coating used is the aleic modified ester gum. Other release coatings may be used; for example, gelatine may be used which is washed away with warm water. Polyvinyl butyral may be used which is washed away with a solvent.

The metallized surface of the resin is removed by scraping, with preliminary soaking in xylol or other solvents for the resin," after which the scrapings are added to xylol for thorough agitation and then a base mirror-flake paste is obtained. Upon agitating the mixture of broken mirrormetal plate particles, the particles are further broken up to a spraying viscosity.

The flat'surface dimensions of the metal mirror-plate particles vary widely, but in the main they are of a size that can pass readily through a 300 mesh size screen. By continued agitation in xylol or other coating solvents the size of the particles can be made finer than a 350 or a 400 mesh. These very fine particles, especially those finer than a 400 mesh, are preferred for spray applications.

Other runs were made by using a vapor plated cellulose acetate film which had a resin release agent applied on the acetate film followed by vapor plating. The vapor metal mirror was removed by solvent scrubbing with a brush in one method.

Another method was carried out by placing 6 inch x 28 inch strips of the vapor plated cellulose acetate film in a can containing some xylol, closing the can and allowing solvent vapors to reflux, thus releasing the metallized film from the acetate film surface. broke up into particles. removed and the mixture of metal mirror-plates and xylol was added to a pigmented synthetic enamel composition; Multiple coats, each separated by a release agent, may

be formed by vapor deposition as hereinabove described, to provide a larger production of the metal mirror-plate particles.

(B) Electroplating of chromium about 10 to 20 millionths of an inch, using a CrO con centration of 33 oz. per gallon at 25 C., and S0 concentration of 0.33 oz. per gallon, a cathode current density of 0.50 to 0.075 ampere per square inch and a time of about 4 to 6 minutes. The brightness of the chromium may be adjusted by varying the current density and the time in accordance with well known practice, see U. S. 1,802,463.

Improvement in producing flake from the film deposited on the moving belt may be realized by reversing the direction of flow between successive depositions, and plural electrodeposited films which separate from each other, each in the desired thickness, may be obtained. The period of time of reversal is about the period of time for deposition, i. e. about 3 seconds. A further improvement is obtained by coating the belt with mineral oil or graphite, etc., to form a metal-releasing, intermediate coating which facilitates the stripping operation. This coating may be applied between the current reversal operations described in the preceding paragraph. The chromium mirror-plate particles produced are highly reflective (upwards of 70%) and readily dispersible in the filmforming vehicles in accordance with the invention.

In accordance with the invention the chromium film is preferably deposited on a synthetic resin metal-releasing, intermediate coating to facilitate stripping, since the mineral oil or graphite releasing coating, although it produces satisfactory particle thicknesses of about 20-25 millionths of an inch, does not provide a sufficiently thin film and a sufiiciently uniform film because of the difliculties involved in providing a smooth uniformly flat surface of the mineral oil or graphite. The synthetic release film as described in the aluminum panel coatings which were vacuum plated, are preferred. In order to provide a smooth highly polished surface to the The metallized film- The cellulose acetate film was electrodepositedrelease coating, synthetic resin or modified natural resin such as. ester gum, it may be pressed with a releasable,

transparent,.highly polished film such as cellophane. or y they are broken up quite easily by simply agitating the liquid and the particles may {be screened in the liquid by;

known classifying methods. v 1

If desired, the particles may be washed to remove'a part or all of the synthetic resin releasing material although it is economical to utilize the synthetic resin releasing agent as a component of a paint, enamel, lacquer or other formulation. 7

The invention therefore contemplates the preparation of the desired metal particles in an inert liquid carrier as well as .with synthetic ,resin film-forming ingredients,

which liquid preparations are particularly useful for the paint industry. These preparations are not to be confused with the coating compositions described and claimed in my above-identified copending application.

The reversing of the current mentioned in the foregoing example is believed to bring about a brightening of the film preparation due to the phenomenom of electropolishing. The electroplating maybe carried out at higher temperatures but the plating proceeds quite rapidly to pro duce undesirablev thicknesses. Accordingly, the lower temperature is preferred and the lower current densities are likewise preferred. I I

Desirable color effects are inherently produced in the metal particles of the specified thickness, due to interference colors. Additional color efiects may be used by treating the metal particles by metal. toning procedures.

Dyes can be introduced to change the shade of the bright aluminum to gold or practically any other color or shade that may be desired. While it is easily possible to apply metals other than aluminum by the same method, the characteristics of aluminum lend themselves best to vapor depositing production operations. For that reason aluminum is preferred unless some special purpose may require the use of another metal. For example to reflect infrared (heat) radiation, gold may be used.

I claim:

l. The method of producing transparent mirror-flake particles particularly adapted for use as metallized flakes in a metallized coating containing a film-forming material. comprising depositing a high light-reflecting metal in a continuous film having a uniform thickness within the range of about 3 millionths of an inch to about 25 millionths of an inch on a base coated with a layer which can be softened or dissolved when contacted with a liquid inert to said metal, treating the base with the said metal film thereupon with said inert liquid to release said metal film from said base and agitating said inert liquid to release said metal film from said base and to comminute the released metal film to thereby produce flakes which are transparent in visible light.

2. A method as claimed in claim 1 wherein'said metal film is vapor deposited under high vacuum and said metalreleasing layer is a synthetic resin layer.

3. A method as claimed in claim 1 wherein said metal film is electrolytically deposited.

4. A method as claimed in claim 1 wherein said film is chromium.

5. A method as claimed in claim 1 wherein said metal film is aluminum.

metal 6. A method as claimed in claim 1 wherein the metal filmis chromium electrodeposited in a thickness of about 10-20 millionths of an inch under current reversal conditio-ns wherein the plating current is reversed for a period of about ,6 of the time which is required for producing the required thickness.

7. A-method as claimed in claim 3 wherein said metal film is chromium.

'8. The method of producing transparent mirror-flake particles particularly adapted for use as metallizing flakes in a metallized coating containing a film-forming material, comprising depositing a high light-reflecting metal in a continuous film having a uniform thickness within the range of about 5 millionths of an inch to about 15 millionths of an inch on a base coated with a layer which can be softened or dissolved when contacted with a liquid inert to said metal, treating the base with the said metal film thereupon with said inert liquid torelease said metal film from said base and agitating said inert liquid to comminute the released metal film to thereby produce flakes which are transparent in visible light and which have a size .sufiiciently small to pass through a 300 mesh screen.

9. A method as claimed in claim 8 wherein said metal film is aluminum.

l0. Mirror-plate metal particles of about 3 to 25 millionths of an inch in thickness in a graded particle size of from about mesh to about 400 mesh in a liquid carrier which is substantially inert to said particles, said particles having a light reflectivity of atleastabout 35% and alight transmission of at least about 0.5% under visible light, and said liquid carrier being adapted to serve as a thinner for coating formulations.

11. Mirror-plate particles in a liquid carrier as claimed in claim 10 wherein said particles are aluminum having a thickness of about 5 to 15 millionths of an inch and said liquid is a solvent for coating compositions of the class consisting of paints, enamels and lacquers.

12. Mirror-plate particles in a liquid carrier as claimed in claim 11 wherein said particles are chromium having a thickness of about 10 to 20 millionths of an inch and said liquid is-a solvent for coating compositions of the class consisting of paints, enamels and lacquers.

13. The method of-producing transparent mirror-flake particles particularly adapted for use as metallizing flakes in a metallized coating containing a film-forming material, comprising depositing a high light-reflecting metal in a continuous film having auniform thickness Within the range of about 5 millionths of an inch to about 15 millionths of an inch on a base coated with a layer which can be softened or dissolved when contacted with a liquid inert to said metal, coating the metal film with a thin continuous smooth coating which can be softened or dissolved when contacted with a liquid inert to said metal, depositing a further film of high light-reflecting metal having a uniform thickness within the range of about 5 millionths of an inch to about 15 millionths of an inch upon said lastnamed coating, treating the base with the said metal films thereupon with said inert liquid to release said metal films from said base and agitating said inert liquid to comminute the released metal films to thereby produce flakes which are transparent in visible light.

References Cited in the file of this patent UNITED STATES PATENTS 

1. THE METHOD OF PRODUCING TRANSPARENT MIRROR-FLAKES PARTICLES PARTICULARLY ADAPTED FOR USE AS METALLIZED FLAKES IN A METALLIZED COATING CONTAINING A FILM-FORMING MATERIAL. COMPRISING DEPOSITING A HIGH LIGHT-REFLECTING METAL IN A CONTINUOUS FILM HAVING A UNIFORM THICKNESS WITHIN THE RANGE OF ABOUT 3 MILLIONTHS OF AN INCH TO ABOUT 25 MILLIONTHS OF AN INCH ON A BASE COATED WITH A LAYER WHICH CAN BE SOFTENED OR DISSOLVED WHEN CONTACTED WITH A LIQUID INSERT TO SAID METAL, TREATING THE BASE WITH THE SAID METAL FILM THEREUPON WITH SAID INSART LIQUID TO RELEASE SAID METAL SAID METAL FILM FROM SAID BASE AND TO COMMINUTE THE RELEASE METAL FILM TO THEREBY PRODUCE FLAKES WHICH ARE TRANSPARENT IN VISIBLE LIGHT. 